The invention relates to a gas analyser comprising a measuring chamber, a gas inlet channel for supplying gas to the measuring chamber, a gas outlet channel for removing gas from the measuring chamber, means for providing a predetermined volume flow rate through the measuring chamber, a device for affecting the gas in the measuring chamber with pulsating magnetic or electromagnetic energy of predetermined pulse rates so as to generate acoustic pressure fluctuations therein, the inlet channel and the outlet channel each comprising an acoustic filter in form of channel sections with reduced sectional area of flow so as to provide a predetermined flow resistance, and a plurality of cavities of a predetermined volume associated therewith.
The gas analyser of the above type may either be formed as a so-called paramagnetic gas analyser employed for measuring the oxygen content in a gas or as a so-called photoacoustic gas analyser, which hereinafter is denoted as PGA (photoacoustic gas analyser), employed for measuring the incidence of one or more specific gasses in a gas mixture.
In a paramagnetic electromagnetic gas analyser the affecting device is an electromagnet affecting the gas in the chamber with a pulsating magnetic field. As oxygen in practice is the only occurring gas, which is paramagnetic, the pulsating magnetic field generates pressure changes in the gas in the measuring chamber depending on the oxygen portion in the gas. These pressure changes are detected by means of a microphone.
In a photoacoustic gas analyser is the affecting device is an electromagnetic radiation source affecting the gas in the measuring chamber with electromagnetic radiation, eg. infrared light. The energy from the light source is periodically absorbed by gas mixture and causes a periodic heating resulting in a corresponding increase in the pressure of the gas. The gas is cooled between the light pulses, whereby the pressure in the chamber decreases correspondingly. These pressure changes are detected by means of a microphone connected to the chamber. Due to the composition of the individual gas mixture constituents the absorption wavelength thereof differs from one another. If the wavelength of the modulated light is set to be close to one of these absorption wavelengths, the rise in temperature and thus the pressure in the chamber increases as the portion of the gas constituent having the absorption wavelength in question increases.
Both in a paramagnetic and in a photoacoustic gas analyser the accuracy of the measurement is conditional on the measuring chamber being kept closed during the measuring process in the sense that the produced pressure fluctuations are retained in the chamber and external noise is prevented from entering the chamber. For instance in connection with a photoacoustic gas analysis this may be obtained by allowing a measured amount of gas to enter the measuring chamber prior to a measurement and subsequently closing off the measuring chamber during the measurement per se.
This measuring method is known from U.S. Pat. No. 4,818,882 and used in the device entitled Multi-gas Monitor, type 1302, from the company of Innova Air Tech Instruments. The said measuring method is, however, encumbered by the drawback that the measurement is intermittent and the measuring time is relatively long, typically of 30-60 seconds. As some physiological measurements require a response time of typically 0.1 second, this measuring method is unsuitable for such applications. Another measuring method is to employ so-called acoustic filters to prevent noise from reaching the measuring chamber and affecting the measuring resultsxe2x80x94at least in the frequency area corresponding to the photoacoustic frequencies. Such acoustic filters allow for a constant volume flow rate through the measuring chamber and is described in the above U.S. Pat. No. 4,818,882.
The said acoustic filters may be formed of narrow flow channels or restrictions providing resistance to gas flow therethrough and of cavities communicating therewith attenuating pressure changes in the gas flow. In order to facilitate the calculation of acoustic filters, the restrictions and the cavities can be equivalent to the electric resistances and capacities, respectively, as the differential equations used for the two systems also are equivalent. Pressure and volume flow rate thus correspond to electric voltage and current, respectively. The cavities may communicate with the flow channels such that the gas flows through said cavities. If, however, a quick response time is required, it is advantageous in relation to the inlet channel to connect the cavities to the flow channel via lateral branches such that the inflowing gas does not flow through the cavities and thereby is mixed with the gas therein.
The number and size of the restrictions and the cavities is determined by the desired physical size of the system and of the frequencies to be dampened by the acoustic filter. Long, thin restrictions offer more resistance to flow than short, wide restrictions and large cavities dampen lower frequencies than small cavities. For obtaining sufficient damping, an acoustic filter may comprise several successive parts formed of a restriction and a cavity, respectively.
The device known as Anaesthetic Gas Monitor Type 1304 from the company of Innova Air Tech Instruments, discloses a PGA comprising acoustic filters of the above type, in which the restrictions of the acoustic filters are formed of thin needle tubes and each cavity of a metal container with a hose connector communicating with the cavity of the container. The needle tubes and the container connectors are interconnected by means of a short silicone hose. One drawback of such a structure is that it is relatively sensitive to vibrations. Yet another drawback is that the structure comprises a large number of components resulting in an expensive and time-consuming manufacture and assembling thereof. Furthermore, in needle tubes with a circular inner cross section, the flow resistance is inversely proportional to the radius in the fourth power. Consequently deviations from the nominal radius result in a quadruple relative deviation of the flow resistance from the nominal value thereof.
A need thus exists for a gas analyser with acoustic filters, which is less sensitive to vibrations and which is more simple to manufacture than the known gas analysers.
The gas analyser according to the invention is characterised in that it comprises at least one sandwich device formed of joined plate-like elements, the channel sections with reduced flow area of the inlet channel and/or the outlet channel being defined by at least two elements and the cavities also being defined by at least two elements, the connections between the respective channel and the respective cavities being provided as transverse apertures in the elements or element portions adjoining the channel and the cavities, respectively.
Since the sandwich device is a mechanically rigid structure, which is insensitive to vibrations, a photoacoustic gas analyser, which is less sensitive to vibrations than known gas analysers, is obtained. Furthermore it is comparatively simple to manufacture and assemble the relatively few elements of the sandwich device. In addition to the acoustic filter it is, moreover, also possible to incorporate other flow channels of the analyser into the sandwich device. The sandwich device thus enables the provision of a very compact gas analyser. Furthermore the fundamental structure of the sandwich device, wherein flow channels are arranged at different levels, is comparable with the structure of printed circuit boards.
According to a preferred embodiment of the invention, the acoustic filters of the inlet channel and the outlet channel may be provided in the same sandwich device so as to obtain a simple and compact embodiment.
Moreover according to the invention the channel or channels of the sandwich device may be defined by a non-through-going groove in a first element and an adjoining second element, in which the transverse apertures connecting the channel with the cavities is formed.
Furthermore according to the invention the channel or channels of the sandwich device may be defined by a through-going slit in a first element and two further adjacent elements, one provided on either side of the first element, the transverse apertures connecting the channel with the cavities being formed in one of the further elements.
According to a preferred embodiment of the invention the first element may be formed of a metal foil having a thickness of between 0.05 and 0.5 mm, preferably 0.1 and 0.2 mm, the width of the slit exceeding the foil thickness, preferably by at least two to three times. As the metal foil thus preferably is rather thin compared to the width of the slit, the width of the channel somewhat exceeds its height, whereby a deviation from the nominal height of the channel only results in the triple deviation of the flow resistance from its nominal value. Since metal foils of very accurate thickness tolerances are readily available, it is expected that a higher degree of accuracy of the flow resistance may be obtained by use thereof than by use of needle tubes. The slits in the metal foils are formed in an accurate processing method, preferably an etching method, so as to prevent burrs on the element in question.
Furthermore according to the invention the transverse apertures may be formed in the same element as the cavities, the cavities being defined by a separate plate-like element on the side opposite the transverse apertures.
Moreover according to the invention the cavities may be formed as recesses in a separate element abutting the elements comprising the transverse apertures.
Furthermore according to the invention each of the cavities associated with the acoustic filter of the inlet channel may communicate with the acoustic filter of the outlet channel via a shunt channel. The shunt channel provides a minor partial flow through the cavities associated with the acoustic filter of the inlet channel and bypassed of the measuring chamber to ensure that a gas interchange between the gas in the inlet channel and the gas in the associated cavities is not effected. Such a gas interchange would have an adverse effect on the response time and the measuring accuracy. The shunt channel is typically dimensioned such that the volume flow rate therethrough is in the order one tenth of the volume flow rate through the measuring chamber.
Furthermore according to the invention the shunt channel may be provided partly by means of at least two plate-like elements of the sandwich device defining shunt channel sections, said elements preferably differing from the elements defining the inlet channel, the outlet channel and the cavities, respectively, and partly by transverse apertures connecting the shunt channel sections with the cavities in the acoustic filter of the inlet channel and with the acoustic filter of the outlet channel, respectively.
Finally according to the invention substantially all of the flow channels of the gas analyser may be provided in elements of the sandwich device. As a result a very compact and rigid structure is obtained which is insensitive to vibrations.